ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-12-10989-2012The mixing state of carbonaceous aerosol particles in northern and southern California measured during CARES and CalNex 2010CahillJ. F.1SuskiK.1SeinfeldJ. H.2ZaveriR. A.3PratherK. A.141Dept. of Chemistry and Biochemistry, University of California San Diego, CA, USA2Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA3Atmpspheric Sciences & Global Division, Pacific Northwest National Laboratory, Richland, WA, USA4Scripps Institute of Oceanography, University of California San Diego, La Jolla, CA, USA2111201212221098911002This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://www.atmos-chem-phys.net/12/10989/2012/acp-12-10989-2012.htmlThe full text article is available as a PDF file from https://www.atmos-chem-phys.net/12/10989/2012/acp-12-10989-2012.pdf

Carbonaceous aerosols impact climate directly by scattering and absorbing
radiation, and hence play a major, although highly uncertain, role in global
radiative forcing. Commonly, ambient carbonaceous aerosols are internally
mixed with secondary species such as nitrate, sulfate, and ammonium, which
influences their optical properties, hygroscopicity, and atmospheric
lifetime, thus impacting climate forcing. Aircraft-aerosol time-of-flight
mass spectrometry (A-ATOFMS), which measures single-particle mixing state,
was used to determine the fraction of organic and soot aerosols that are
internally mixed and the variability of their mixing state in California
during the Carbonaceous Aerosols and Radiative Effects Study (CARES) and the
Research at the Nexus of Air Quality and Climate Change (CalNex) field
campaigns in the late spring and early summer of 2010. Nearly 88% of all
A-ATOFMS measured particles (100–1000 nm in diameter) were internally mixed
with secondary species, with 96% and 75% of particles internally mixed
with nitrate and/or sulfate in southern and northern California,
respectively. Even though atmospheric particle composition in both regions
was primarily influenced by urban sources, the mixing state was found to vary
greatly, with nitrate and soot being the dominant species in southern
California, and sulfate and organic carbon in northern California.
Furthermore, mixing state varied temporally in northern California, with soot
becoming the prevalent particle type towards the end of the study as regional
pollution levels increased. The results from these studies demonstrate that
the majority of ambient carbonaceous particles in California are internally
mixed and are heavily influenced by secondary species that are most prevalent
in the particular region. Based on these findings, considerations of
regionally dominant sources and secondary species, as well as temporal
variations of aerosol physical and optical properties, will be required to
obtain more accurate predictions of the climate impacts of aerosol in
California.